Technical Field
This disclosure relates to the field of nanotechnology, and more specifically to processes of controlling silver nanowire morphology, i.e., their lengths, diameters and aspect ratios.
Description of the Related Art
Silver nanowires are the key components of transparent conductive films, which find wide applications in electronic devices, such as touch panels, photovoltaic cells, flat liquid crystal displays (LCD), organic light emitting diodes (OLED), and the like. Unlike transparent conductive films formed by vacuum depositing a continuous metal oxide layer (e.g., indium tin oxide), metal nanowires can form a conductive film of networking nanowires by a solution-based coating process.
The electrical and optical properties of a transparent conductive film thus formed are dependent on the morphology of the constituent nanowires, i.e., the length and diameter, as well as aspect ratio (length/diameter). Generally, nanowires of high aspect ratios form a more efficient conductive network by allowing a lower density of wires to achieve higher transparency for a given film resistivity. More specifically, because each nanowire can be considered a conductor, individual nanowire's length and diameter can affect the overall network conductivity and as such, the final film conductivity. For instance, as nanowires get longer, fewer may be needed to form a conductive network. Likewise, the diameters of the nanowires also have a direct effect on film conductivity because the thinner the diameter, the higher the resistivity (or the lower the conductivity). Thus, a resulting film may be less conductive for a given silver level.
The lengths and diameters of the silver nanowires also impact the optical transparency and light scattering (haze) of the resulting transparent conductive film. Nanowire networks are optically transparent because the constituent nanowires make up a very small fraction of the film. Nevertheless, the nanowires absorb and scatter light, the extents of which are affected by the lengths and diameters of the constituent nanowires. Generally, thinner nanowires scatter less light, resulting in better optical properties of the film. Furthermore, the optical property is impacted by low aspect ratio nanostructures (a byproduct of the synthesis process) which scatter light without contributing to the conductivity of the network.
U.S. Pat. Nos. 8,454,721 and 8,709,125, in the name of Cambrios Technologies Corporation, are directed to a “polyol” synthesis for preparing silver nanowires. The process involves reacting a silver salt (e.g., silver nitrate), a capping agent (e.g., polyvinyl pyrrolidone), and a chloride source (e.g., quaternary ammonium chloride) in a polyol solvent (e.g., ethylene glycol or propylene glycol). The polyol solvent serves as a reducing agent that reduces the silver salt to elemental silver, whereas the other reactants facilitate the axial growth of the crystalline silver into silver nanowires.
U.S. Published Application Nos. 2011/0174190 and 2013/0291683, in the name of Cambrios Technologies Corporation, are directed to a modified polyol synthesis by which thinner nanowires can be prepared. In particular, the silver salt is added in two stages during the reaction. The resulting silver nanowires have mean diameters of about 26-32 nm and a standard deviation in the range of 4-6 nm; and a mean length of about 10-22 μm.